Kinda up to interpretation but usually crunchy is when someone has melted their motor insulation and the motor is irreparable.

You're supposed to feel the "clunks" of the motor. A worn-out rotor / overheated one will feel like less strong clunks (I can't think of a better word). It's when the rotor loses its magnetic strength, usually from heat, and this can be over time (so older motors might need new rotors even if they never had a massive overheat). I don't remember what the exact temp is but I think it's like 170F or something like that when the rotor starts losing its magnetic strength.

When you rotate a Bl motor by hand, you can fell the rotor magnet 'grab' the metal and in a way, 'clunk' into place. This is just magnetics at work. But it can feel like a clunk or hard click when moving them by hand.

Well, if more information was available on the stators it would make it more easy to calculate inductance of the rotor without taking it apart.
However it would just be extremely easy to turn the rotor shaft by hand if the rotor past its maximum curie temp too long and or no gauss.

I have the problem of pretty hard clunks. Advice?

hard clunks are good, soft clunks are bad (means the rotor lost it's magnetic strength) is the simple way to put it.

Does it feel like your rolling a ball in sand, grinding? Or does it feel smooth, but like your lifting 200 lbs with your fingers up over a hill?

If it doesn't feel like its grinding, aka its smooth feeling but seems like your pushing that weight over the hill crest, that is normal.

If it feels like its grinding or like a ball rolling on glass, with sand on it, that's bad. lol

I've seen a few stator failures where a portion of the winding came loose creating an intermittent or permanent open circuit. I'm guessing a soldered or welded connection gave way, likely from vibration and impacts. Not common but it can happen. Or excessive heat can damage the insulation.

Brushed motors too. Frequently called cogging I believe.

Cogging happens at repeatable intervals as the shaft is turned, the same number of times each revolution, etc.

I'd first check the bearings.

Kinda. Cogging is when a motor is starting up under low rpms and it bouces from forward and reverse or is very jerky. That is cogging.

But, yes, brushed motors do it too. Its the magnets in the rotors aligning to the heaviest area of the stators where the metal windings meet.
This creates the strongest magnetic forces, thus the strongest pull, or clunk, as the magnetic force 'grabs' onto the metal windings.
That is what an electric motor is made of, just brushed motors generally have very weak magnetic pull until its under load.
Where BL motors have it all the time.

Hey hey. I replaced the rotor in one of my motors. It runs better. Not sure if this is widely known. So I thought I would share. Anyways spec rotors are 7.2 core. Mod rotors are 5.0 or 6.0 core. mod rotors are not roar legal in spec classes. Found these cruising web. https://www.ebay.com/itm/C50135-1-10...ed/7632-2357-0. And more info. Seems like a good upgrade for older motors that the rotor is weak. Cheers. https://www.rccaraction.com/brushless-rotors-explained/
. Edit update . So when item is not available in future . They claim it fits many mod motors. They have different sizes.

• Fits Rocket Series Modified Motors as well as Trinity, Fantom, R1, Motiv and many other brands with similar rotor designs. Proper shimming is required with each rotor and motor.

It's not ROAR legal to change to a rotor that's not approved for your motor, which is widely known.

Roar lol outdated old guys.

Not sure what's outdated with a "Spec motor needing a spec rotor" rule.

Rotors are permanent magnets. Early type rotors were laminated. Rotor now solid magnets. What kills them is Heat. Monitor the heat, take note of heat, gear ratio, and timing. Then decide on replacing